a) Field of the Invention
The invention relates to a video decoding apparatus, a video decoding method and a digital audio/video playback system and, more particularly, to a video decoding apparatus, a video decoding method and a digital audio/video playback system capable of controlling presentation of sub-pictures.
b) Description of the Related Art
Due to the advance of information technology, both audio and video data can be stored, transmitted and played back in digital format. Besides providing movies of better quality, audio and video data in digital format offers many additional functions. Take the digital versatile disc (DVD) for instance, not only can the DVD store high-quality movies, but the DVD can also store multilingual audio tracks and multiple sub-pictures for users to choose from during playback. One of the major functions of sub-pictures is to act as subtitles, for example, an English film can include therein English and Chinese audio tracks, and English and Chinese subtitles, and during playback, users can choose the English audio track with Chinese subtitle, or the English audio track with English subtitle, as well as the Chinese audio track with English or Chinese subtitles. Therefore, this function facilitates internationalization of films.
As common practice, even though the DVD contains multiple subtitles, the DVD is usually played back with only one subtitle displayed. If two or more subtitles are displayed simultaneously, there can be overlay among the subtitles, making them difficult to read, or only one of the subtitles is displayed in the overlaid region such that other subtitles cannot be completely displayed.
Furthermore, differently formatted DVD films can cause incomplete display of the subtitle during playback. For example, in Phase Alternate Line (PAL) standard adopted by most European countries and Australia, the resolution of the DVD film is 720×576 pixels. In National Television System Committee (NTSC) standard adopted by North America and Japan, the resolution of the DVD film is 720×480 pixels. Therefore, when a DVD film recorded according to PAL standard is played back in a playback system compliant with NTSC standard, there is a possibility that the subtitle at the lower end of the picture is cut off and cannot be displayed completely. In addition, similar problems exist when playback systems with different formats, namely 4:3 or 16:9, are used to play differently formatted DVD films.
In summary, it would be desirable if the sub-pictures can be completely displayed and overlay among multiple sub-pictures can be avoided during playback of a digital film.